The present invention relates to a particle charging device and an electric dust collecting apparatus especially suitable for charging and collecting a high resistance dust, in which charging of particles is performed by means of a periodically varying voltage such as a repetitive pulse voltage and the like. In a single stage type of electric dust collecting apparatus in which a charging section for charging particles and a collector section for collecting charged particles under an electric field exist within the same space, it is known that by applying a repetitive pulse voltage to discharge electrodes it becomes possible to raise a spark discharge voltage and to prevent an inverse ionization phenomenon. Furthermore, as a method for practicing the aforementioned improvement, a novel system is described in pending U.S. application entitled PARTICLE CHARGING DEVICE FOR USE IN AN ELECTRIC DUST COLLECTING APPARATUS, Ser. No. 460,762 filed on Apr. 15, 1974. In this system, cylindrical third electrodes are provided in the proximity of each discharge electrode, a high voltage that is about to generate a spark discharge being applied between said third electrodes and dust collecting electrodes, a group of ions being produced impulsively by applying a repetitive pulse voltage between said third electrodes and said disharge electrodes, and said group of ions are directed towards the dust collecting electrodes to achieve the charging of electrodes.
In the conventional electric dust collecting apparatus, generally a corona current density is uniquely determined by an electric field in a dust collecting space, while driving of particles is achieved by a large electric field that is about to generate a spark discharge and that is established between third electrodes and dust collecting electrodes. However, in case of employing the aforementioned repetitive voltage pulse, an ion current density i (A/m.sup.2) can be varied in accordance with the repetition frequency of the pulse voltage regardless of the electric field, and therefore, control can be made on the ion current density in such manner that as the value of the specific resistance .rho..sub.d (.OMEGA.-m) of the accumulated dust layer on the dust collecting electrodes is varied the relation of i .times. .rho..sub.d &lt; E.sub.b (where E.sub.b represents a breakdown electric field intensity of the dust layer, which amounts to about 10,000 V/m) may be retained, in other words, in such manner that the breakdown of the dust layer may be prevented at all times and thereby occurance of inverse ionization phenomena caused by the breakdown can be suppressed.
In addition, since the group of ions generated impulsively in this case are strongly expanded and dispersed owing to a Coulomb's repulsive force and thus they are distributed uniformly over the dust collecting electrodes, said ion current density i becomes uniform over the all positions, and from this aspect also the occurrence of innverse ionization phenomena at a position where the ion current density i is locally increased can be prevented. Thus, the aforementioned system employing a repetitive voltage pulse is favorable. However, in order to effectively practice the aforementioned system, it must be assured that only upon applying a pulse voltage is the discharge from the discharge electrode realized and during the period the pulse voltage is not applied the discharge electrode is completely shielded by the third electrodes resulting in no discharge current. Otherwise, the ion current flows in a D.C. mode, and it becomes impossible both to control the magnitude of the ion current regardless of the electric field and to make the distribution of the ion current uniform, so that the advantages of the impulse type particle charging system would be lost.
In a practical dust collecting apparatus, it is necessary in view of machining tolerances that the distance between a discharge electrode and an adjacent third electrode of several centimeters or more be chosen. However, if it is so chosen, the shielding effect of the third electrode is greatly reduced, so that it becomes very difficult to suppress a corona discharge from the discharge electrode when the pulse voltage is not applied. Furthermore, even if the shielding effect should be realized under a particular condition, the temperature and dust concentration of the inlet gas will vary from time to time during operation of the apparatus, and accordingly, said particular condition would not necessarily be satisfied, resulting in unstable operation.